Liposomes have the potential as versatile drug carriers in the bloodstream. Because they contain both an inner aqueous compartment and an outer lipophilic shell, liposomes can be loaded with both water-soluble or lipid-soluble drugs. Since liposomes can be formed of natural biochemical lipid, such as phospholipids and cholesterol, they can be broken down by the body without toxic side effects (Poznansky).
Until recently, the potential of liposomes as drug carriers has been limited by the rapid clearance of liposomes from the bloodstream. For example, conventional liposomes may be largely cleared from the bloodstream within 1-2 hours after intravenous administration (Saba).
A variety of approaches for extending the circulation time of liposomes have been proposed. Two of these have been successful in extending the halflife of liposomes in the bloodstream by periods of up to 40-50 hours. In one approach, described in co-owned U.S. Pat. No. 4,837,028, liposomes are formulated with the ganglioside G.sub.M1 and predominantly rigid lipids. In another general approach, disclosed in co-owned U.S. Pat. No. 5,013,556, liposomes are coated with a layer of polyethylene glycol (PEG) chains.